Multiple transformer charging circuits for implantable medical devices

ABSTRACT

An implantable medical device includes a low-power circuit, a high-power circuit, and a dual-cell power source. The power source is coupled to a dual-transformer such that each cell is connected to only one of the transformers. Each transformer includes multiple windings and each of the windings is coupled to a capacitor, and the capacitors are all connected in a series configuration. The low power circuit is coupled to the power source and issues a control signal to control the delivery of charge from the power source to the plurality of capacitors through the first and second transformers.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/047,122, filed on Sep. 8, 2014. The disclosure of the aboveapplication is incorporated herein by reference in its entirety.

The present application is related to co-pending and commonly-assignedU.S. patent application Ser. No. 14/695,264 which is entitledMulti-Primary Transformer Charging Circuits for Implantable MedicalDevices; U.S. patent application Ser. No. 14/695,309, which is entitledImplantable Medical Devices Having Multi-Cell Power Sources; U.S. patentapplication Ser. No. 14/695,630, which is entitled Transformer-BasedCharging Circuits for Implantable Medical Devices; U.S. patentapplication Ser. No. 14/695,948, which is entitled Implantable MedicalDevices Having Multi-Cell Power Sources; U.S. patent application Ser.No. 14/695,887, which is entitled Transthoracic Protection Circuit forImplantable Medical Devices; and U.S. patent application Ser. No.14/695,826, which is entitled Monitoring Multi-Cell Power Source of anImplantable Medical Device, all of which are filed concurrently herewithand all of which are incorporated herein by reference in theirentireties.

FIELD

The present disclosure relates to body implantable medical devices and,more particularly to circuits and techniques implemented in animplantable medical device to provide an electrical therapeutic output.

BACKGROUND

The human anatomy includes many types of tissues that can eithervoluntarily or involuntarily, perform certain functions. After disease,injury, or natural defects, certain tissues may no longer operate withingeneral anatomical norms. For example, organs such as the heart maybegin to experience certain failures or deficiencies. Some of thesefailures or deficiencies can be diagnosed, corrected or treated withimplantable medical devices.

Implantable medical electrical leads are used with a wide variety ofthese implantable medical devices. The medical leads may be configuredto allow electrodes to be positioned at desired cardiac locations sothat the device can monitor and/or deliver stimulation therapy to thedesired locations. For example, electrodes on implantable leads maydetect electrical signals within a patient, such as anelectrocardiogram, in addition to delivering electrical stimulation.

Currently, ICD's use endocardial or epicardial leads which extend fromthe ICD housing through the venous system to the heart. Electrodespositioned in or adjacent to the heart by the leads are used for pacingand sensing functions. Cardioversion and defibrillation shocks aregenerally applied between a coil electrode carried by one of the leadsand the ICD housing, which acts as an active can electrode.

A subcutaneous implantable cardioverter defibrillator (SubQ ICD) differsfrom the more commonly used ICD's in that the housing and leads aretypically implanted subcutaneously such that the sensing and therapy areaccomplished subcutaneously. The SubQ ICD does not require leads to beplaced in the heart or in contact with the heart. Instead, the SubQ ICDmakes use of one or more electrodes on the housing, together with asubcutaneous lead that carries a defibrillation coil electrode and asensing electrode.

The implantable medical devices are typically battery powered and oftenutilize capacitors or other electrical charge storage components to holdan electrical output to be made available to a patient. Due to thenature of defibrillation therapy or other high voltage therapy, it isnot practical for the implantable medical device to supply the energyupon instantaneous demand by drawing from the power source. Instead,additional circuitry is provided to transfer and store the energy fromthe power source to accumulate a desired voltage level.

However, the placement of the SubQ ICD lead(s) and electrode(s) outsidethe heart presents a challenge to generating sufficient energy levelsthat are required to deliver appropriate therapy. As described herein,the present disclosure addresses the need in art to provide circuitryand techniques for generating appropriate electrical stimulation therapyin a SubQ ICD system.

SUMMARY

In accordance with aspects of this disclosure, circuits and techniquesimplemented in an implantable medical device are provided for generatingan electrical stimulation therapy from a multi-cell power source. Suchelectrical stimulation therapy exhibits an output having a highervoltage than the voltage available directly from the battery or a highercurrent than the current available directly from the battery.

In accordance with some embodiments, the implantable medical deviceincludes (a) a first transformer having a first magnetic core, a firstprimary winding disposed around the core and a first plurality ofsecondary windings disposed around the first core and magneticallycoupled to the first primary winding, (b) a second transformer having asecond magnetic core, a second primary winding disposed around thesecond core; and a second plurality of secondary windings disposedaround the second core and magnetically coupled to the second primarywinding, (c) a power source having at least a first cell and a secondcell, the first cell being coupled to the first primary winding and thesecond cell being coupled to the second primary winding, (d) a pluralityof capacitors including a first set of capacitors coupled in a seriesconfiguration, with each of the first set of capacitors being coupled toa single winding of the first plurality of secondary windings, and asecond set of capacitors coupled in a series configuration, with each ofthe second set of capacitors is coupled to a single winding of thesecond plurality of secondary windings, such that the first set ofcapacitors and the second set of capacitors are coupled together in aseries configuration, and (e) a low power circuit coupled to the powersource for controlling the delivery of charge from the power source tothe plurality of capacitors through the first and second transformers.

In further aspects of the embodiments of the present disclosure, the lowpower circuit includes a first resistive voltage divider coupled acrossthe first set of capacitors, a second resistive voltage divider coupledacross the second set of capacitors, a charge monitoring circuit coupledto each of the first and second resistive voltage dividers, a firstswitching element coupled along a first current path of the first celland the transformer, and a second switching element coupled along asecond current path of the second cell and the transformer.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of particular embodiments of thepresent disclosure and therefore do not limit the scope of thedisclosure. The drawings are not to scale (unless so stated) and areintended for use in conjunction with the explanations in the followingdetailed description. Embodiments will hereinafter be described inconjunction with the appended drawings wherein like numerals/lettersdenote like elements, and:

FIG. 1 is a front view of a patient implanted with an implantablecardiac system;

FIG. 2 is a side view the patient implanted with an implantable cardiacsystem;

FIG. 3 is a transverse view of the patient implanted with an implantablecardiac system;

FIG. 4 depicts a schematic diagram of an embodiment of operationalcircuitry included in an implantable cardiac defibrillator of thecardiac system of FIGS. 1-3;

FIG. 5 illustrates an exemplary schematic diagram showing a portion ofthe operational circuitry of FIG. 4 in accordance with an embodiment ofthe disclosure; and

FIG. 6 illustrates an exemplary schematic diagram showing a portion ofthe operational circuitry of FIG. 4 in accordance with an embodiment ofthe disclosure.

DETAILED DESCRIPTION

FIG. 1 is a conceptual diagram of a patient 12 implanted with an exampleextravascular cardiac defibrillation system 10. In the exampleillustrated in FIG. 1, extravascular cardiac defibrillation system 10 isan implanted subcutaneous defibrillation system for purposes ofillustration.

Extravascular cardiac defibrillation system 10 includes an implantablemedical device such as implantable cardiac defibrillator (ICD) 14connected to at least one implantable cardiac defibrillation lead 16.ICD 14 of FIG. 1 is implanted subcutaneously on the left side of patient12. Defibrillation lead 16, which is connected to ICD 14, extendsmedially from ICD 14 toward sternum 28 and xiphoid process 24 of patient12. At a location near xiphoid process 24 defibrillation lead 16 bendsor turns and extends subcutaneously superior, substantially parallel tosternum 28. In the example illustrated in FIG. 1, defibrillation lead 16is implanted such that lead 16 is offset laterally to the left side ofthe body of sternum 28 (i.e., towards the left side of patient 12).

ICD 14 may interact with an external device 4 such as a patientprogrammer or a clinician programmer via a 2-way telemetry link. Such aprogrammer communicates with ICD 14 via telemetry as is known in theart. The programmer 4 may thereby establish a telemetry session with ICD14 to provide programs, instructions, parameters, data, and otherinformation to ICD 14, and to likewise receive status, data, parameters,programs, and other information from the ICD 14. Status informationreceived from the ICD 14 may include data about the remaining longevityof the power source (e.g., a battery) based on the amount of charge thathas thus far been delivered by the battery and consumed by the ICD 14 ascompared to when the battery was in the full-charged state (“batterycapacity”). Status information may also include an “Elective ReplacementIndicator” (ERI) to indicate when surgery must be scheduled to replaceICD 14. Status may also include an “End of Life” (EOL), which isactivated to signify end-of-battery life.

Defibrillation lead 16 is placed along sternum 28 such that a therapyvector between defibrillation electrode 32 and a second electrode (suchas a housing or can electrode 36 36 of ICD 14 or an electrode placed ona second lead) is substantially across the ventricle of heart 26. Thetherapy vector may, in one example, be viewed as a line that extendsfrom a point on the defibrillation electrode 32 to a point on thehousing or can electrode 36 of ICD 14. In another example,defibrillation lead 16 may be placed along sternum 28 such that atherapy vector between defibrillation electrode 32 and a housing or canelectrode 36 of ICD 14 (or other electrode) is substantially across anatrium of heart 26. In this case, extravascular ICD system 10 may beused to provide atrial therapies, such as therapies to treat atrialfibrillation.

The embodiment illustrated in FIG. 1 is an example configuration of anextravascular ICD system 10 and should not be considered limiting of thetechniques described herein. For example, although illustrated as beingoffset laterally from the midline of sternum 28 in the example of FIG.1, defibrillation lead 16 may be implanted such that lead 16 is offsetto the right of sternum 28 or over sternum 28. Additionally,defibrillation lead 16 may be implanted such that it is notsubstantially parallel to sternum 28, but instead offset from sternum 28at an angle (e.g., angled lateral from sternum 28 at either the proximalor distal end). As another example, the distal end of defibrillationlead 16 may be positioned near the second or third rib of patient 12.However, the distal end of defibrillation lead 16 may be positionedfurther superior or inferior depending on the location of ICD 14,location of electrodes 32, 34, and 30, or other factors.

Although ICD 14 is illustrated as being implanted near a midaxillaryline of patient 12, ICD 14 may also be implanted at other subcutaneouslocations on patient 12, such as further posterior on the torso towardthe posterior axillary line, further anterior on the torso toward theanterior axillary line, in a pectoral region, or at other locations ofpatient 12. In instances in which ICD 14 is implanted pectorally, lead16 would follow a different path, e.g., across the upper chest area andinferior along sternum 28. When the ICD 14 is implanted in the pectoralregion, the extravascular ICD system may include a second lead includinga defibrillation electrode that extends along the left side of thepatient such that the defibrillation electrode of the second lead islocated along the left side of the patient to function as an anode orcathode of the therapy vector of such an ICD system.

ICD 14 includes a housing that forms a hermetic seal that protectscomponents within ICD 14. The housing of ICD 14 may be formed of aconductive material, such as titanium or other biocompatible conductivematerial or a combination of conductive and non-conductive materials. Insome instances, the housing of ICD 14 functions as an electrode(sometimes referred to as a housing electrode or can electrode) that isused in combination with one of electrodes 32, 34, or 30 to deliver atherapy to heart 26 or to sense electrical activity of heart 26. ICD 14may also include a connector assembly (sometimes referred to as aconnector block or header) that includes electrical feedthroughs throughwhich electrical connections are made between conductors withindefibrillation lead 16 and electronic components included within thehousing. The housing may enclose one or more components, includingprocessors, memories, transmitters, receivers, sensors, sensingcircuitry, therapy circuitry and other appropriate components (oftenreferred to herein as modules).

Defibrillation lead 16 includes a lead body having a proximal end thatincludes a connector configured to connect to ICD 14 and a distal endthat includes one or more electrodes 32, 34, and 30. The lead body ofdefibrillation lead 16 may be formed from a non-conductive material,including silicone, polyurethane, fluoropolymers, mixtures thereof, andother appropriate materials, and shaped to form one or more lumenswithin which the one or more conductors extend. However, the techniquesare not limited to such constructions. Although defibrillation lead 16is illustrated as including three electrodes 32, 34 and 30,defibrillation lead 16 may include more or fewer electrodes.

Defibrillation lead 16 includes one or more elongated electricalconductors (not illustrated) that extend within the lead body from theconnector on the proximal end of defibrillation lead 16 to electrodes32, 34 and 30. In other words, each of the one or more elongatedelectrical conductors contained within the lead body of defibrillationlead 16 may engage with respective ones of electrodes 32, 34 and 30.When the connector at the proximal end of defibrillation lead 16 isconnected to ICD 14, the respective conductors may electrically coupleto circuitry, such as a therapy module or a sensing module, of ICD 14via connections in connector assembly, including associatedfeedthroughs. The electrical conductors transmit therapy from a therapymodule within ICD 14 to one or more of electrodes 32, 34 and 30 andtransmit sensed electrical signals from one or more of electrodes 32, 34and 30 to the sensing module within ICD 14.

ICD 14 may sense electrical activity of heart 26 via one or more sensingvectors that include combinations of electrodes 34 and 30 and a housingor can electrode 36 of ICD 14. For example, ICD 14 may obtain electricalsignals sensed using a sensing vector between electrodes 34 and 30,obtain electrical signals sensed using a sensing vector betweenelectrode 34 and the conductive housing or can electrode 36 of ICD 14,obtain electrical signals sensed using a sensing vector betweenelectrode 30 and the conductive housing or can electrode 36 of ICD 14,or a combination thereof. In some instances, ICD 14 may even sensecardiac electrical signals using a sensing vector that includesdefibrillation electrode 32, such as a sensing vector betweendefibrillation electrode 32 and one of electrodes 34 or 30, or a sensingvector between defibrillation electrode 32 and the housing or canelectrode 36 of ICD 14.

ICD 14 may analyze the sensed electrical signals to detect tachycardia,such as ventricular tachycardia or ventricular fibrillation, and inresponse to detecting tachycardia may generate and deliver an electricaltherapy to heart 26. For example, ICD 14 may deliver one or moredefibrillation shocks via a therapy vector that includes defibrillationelectrode 32 of defibrillation lead 16 and the housing/can electrode.Defibrillation electrode 32 may, for example, be an elongated coilelectrode or other type of electrode. In some instances, ICD 14 maydeliver one or more pacing therapies prior to or after delivery of thedefibrillation shock, such as anti-tachycardia pacing (ATP) or postshock pacing. In these instances, ICD 14 may generate and deliver pacingpulses via therapy vectors that include one or both of electrodes 34 and30 and/or the housing/can electrode. Electrodes 34 and 30 may comprisering electrodes, hemispherical electrodes, coil electrodes, helixelectrodes, segmented electrodes, directional electrodes, or other typesof electrodes, or combination thereof. Electrodes 34 and 30 may be thesame type of electrodes or different types of electrodes, although inthe example of FIG. 1 both electrodes 34 and 30 are illustrated as ringelectrodes.

Defibrillation lead 16 may also include an attachment feature 29 at ortoward the distal end of lead 16. The attachment feature 29 may be aloop, link, or other attachment feature. For example, attachment feature29 may be a loop formed by a suture. As another example, attachmentfeature 29 may be a loop, link, ring of metal, coated metal or apolymer. The attachment feature 29 may be formed into any of a number ofshapes with uniform or varying thickness and varying dimensions.Attachment feature 29 may be integral to the lead or may be added by theuser prior to implantation. Attachment feature 29 may be useful to aidin implantation of lead 16 and/or for securing lead 16 to a desiredimplant location. In some instances, defibrillation lead 16 may includea fixation mechanism in addition to or instead of the attachmentfeature. Although defibrillation lead 16 is illustrated with anattachment feature 29, in other examples lead 16 may not include anattachment feature 29. In this case, defibrillation lead 16 may beconnected to or secured to an implant tool via an interference fit aswill be described in more detail herein. An interference fit, sometimesalso referred to as a friction fit, is a fastening between two partswhich is achieved by friction after the parts are pushed together,rather than by any other means of fastening.

Lead 16 may also include a connector at the proximal end of lead 16,such as a DF4 connector, bifurcated connector (e.g., DF-1/IS-1connector), or other type of connector. The connector at the proximalend of lead 16 may include a terminal pin that couples to a port withinthe connector assembly of ICD 14. In some instances, lead 16 may includean attachment feature at the proximal end of lead 16 that may be coupledto an implant tool to aid in implantation of lead 16. The attachmentfeature at the proximal end of the lead may separate from the connectorand may be either integral to the lead or added by the user prior toimplantation.

Defibrillation lead 16 may also include a suture sleeve or otherfixation mechanism (not shown) located proximal to electrode 30 that isconfigured to fixate lead 16 near the xiphoid process or lower sternumlocation. The fixation mechanism (e.g., suture sleeve or othermechanism) may be integral to the lead or may be added by the user priorto implantation.

The example illustrated in FIG. 1 is exemplary in nature and should notbe considered limiting of the techniques described in this disclosure.For instance, extravascular cardiac defibrillation system 10 may includemore than one lead. In one example, extravascular cardiac defibrillationsystem 10 may include a pacing lead in addition to defibrillation lead16.

In the example illustrated in FIG. 1, defibrillation lead 16 isimplanted subcutaneously, e.g., between the skin and the ribs and/orsternum. In other instances, defibrillation lead 16 (and/or the optionalpacing lead) may be implanted at other extravascular locations. In oneexample, defibrillation lead 16 may be implanted at least partially in asubsternal location. In such a configuration, at least a portion ofdefibrillation lead 16 may be placed under or below the sternum in themediastinum and, more particularly, in the anterior mediastinum. Theanterior mediastinum is bounded laterally by pleurae, posteriorly bypericardium, and anteriorly by sternum. Defibrillation lead 16 may be atleast partially implanted in other extra-pericardial locations, i.e.,locations in the region around, but not in direct contact with, theouter surface of heart 26. These other extra-pericardial locations mayinclude in the mediastinum but offset from sternum 28, in the superiormediastinum, in the middle mediastinum, in the posterior mediastinum, inthe sub-xiphoid or inferior xiphoid area, near the apex of the heart, orother location not in direct contact with heart 26 and not subcutaneous.In still further instances, the implant tools described herein may beutilized to implant the lead at a pericardial or epicardial locationoutside the heart 26. Moreover, implant tools such as those describedherein may be used to implant non-cardiac leads in other locationswithin patient 12.

In an example, lead 16 may be placed in the mediastinum and, moreparticularly, in the anterior mediastinum. The anterior mediastinum isbounded laterally by pleurae 40, posteriorly by pericardium 38, andanteriorly by sternum 22. Lead 16 may be implanted within themediastinum such that one or more electrodes 32 and 34 are located overa cardiac silhouette of the ventricle as observed via fluoroscopy. Inthe example illustrated in FIGS. 1-3, lead 16 is located substantiallycentered under sternum 22. In other instances, however, lead 16 may beimplanted such that it is offset laterally from the center of sternum22. Although described herein as being implanted in the substernalspace, the mediastinum, or the anterior mediastinum, lead 16 may beimplanted in other extra-pericardial locations.

Electrodes 30, 32, and 34 may comprise ring electrodes, hemisphericalelectrodes, coil electrodes, helical electrodes, ribbon electrodes, orother types of electrodes, or combinations thereof. Electrodes 30, 32and 34 may be the same type of electrodes or different types ofelectrodes. In the example illustrated in FIGS. 1-3 electrode 34 is acoil electrode and electrodes 30 and 34 are ring, or hemisphericalelectrodes.

FIG. 4 is a schematic diagram of operational circuitry 48 included inICD 14 according to an embodiment of the present disclosure. It isunderstood that the system of FIG. 4 includes both low power circuitryand high power circuitry. The present disclosure may be employed in adevice that provides either or both of a high power electricalstimulation therapy, such as a high power defibrillation therapy, or alow power electrical stimulation therapy, such a pacing pulse, or both.Accordingly, the components in the operational circuitry 48 may supportgeneration and delivery of either one or both such therapies. For easeof description, this disclosure will describe an operational circuitry48 that supports only a high power electrical stimulation therapy, suchas cardioversion and/or defibrillation stimulation therapy. However, itshould be noted that the operational circuitry 48 may also providedefibrillation threshold (DFT) induction therapy or post-shock pacingsuch as anti-tachycardia pacing (ATP) therapy.

The operational circuitry 48 is provided with at least one or more powersources 46 which may include a rechargeable and/or non-rechargeablebattery having one or more cells. As used in this disclosure, the term“cell” refers to a battery cell which, as is understood in the art,includes an anode terminal and a cathode terminal. An example of abattery cell is set forth in commonly assigned U.S. Patent ApplicationNo. US 2011/0179637 “Implantable Medical Devices with Low VolumeBatteries, and Systems”, to Norton which is incorporated herein byreference. As described in greater detail below, the power source 46 canassume a wide variety of forms. Similarly, the operational circuitry 48,which includes the low power circuit 60 and the output circuit 56, caninclude analog and/or digital circuits, can assume a variety ofconfigurations, and is electrically connected to the power source 46.

A power source monitoring circuit 62 is provided for monitoring themagnitude of residual energy and/or rate of depletion of energy from thepower source 46. The monitoring circuit 62 may monitor the power sourceby measuring a parameter that is, for example, indicative of theresidual energy or rate of discharge, of the power source 46. Themonitoring circuit 62 may employ techniques that involve computing theindication of the residual energy, or rate of discharge, of the powersource 46 (or individual cells) utilizing a parameter such as thevoltage across terminals of power source 46. In other embodiments,monitoring circuit 62 may alternatively or additionally have thecapability to measure a parameter such as current flowing from the powersource 46.

The output circuit 56 and the low power circuit 60 are typicallyprovided as part of an electronics module associated with the ICD 14. Ingeneral terms, the output circuit 56 is configured to deliver anelectrical pulse therapy, such as a defibrillation or acardioversion/defibrillation pulse. In sum, the output circuit 56 isresponsible for applying stimulating pulse energy between the variouselectrodes 28-34 (FIG. 1) of the ICD 14. As is known in the art, theoutput circuit 56 may be associated with a capacitor bank (not shown)for generating an appropriate output energy, for example in the range of0.1-40 Joules.

The low power circuit 60 is similarly well known in the art. In generalterms, the low power circuit 60 monitors heart activity and signalsactivation of the output circuit 56 for delivery of an appropriatestimulation therapy. Further, as known in the art, the low power circuit60 may generate a predetermined series of pulses from the output circuit56 as part of an overall therapy.

In an embodiment, ICD 14 functions are controlled by means of storedsoftware, firmware and hardware that cooperatively monitor the EGM,determine when a cardioversion or defibrillation shock necessary, anddeliver prescribed defibrillation therapies. The schematic diagram ofFIG. 4 incorporates circuitry set forth in commonly assigned U.S. Pat.No. 5,163,427 “Apparatus for Delivering Single and MultipleCardioversion and Defibrillation Pulses” to Keimel and U.S. Pat. No.5,188,105 “Apparatus and Method for Treating a Tachyarrhythmia” toKeimel, for example, both incorporated herein by reference in theirentireties, for selectively delivering single phase, simultaneousbiphasic and sequential biphasic cardioversion-defibrillationstimulation therapy. In an exemplary implementation, IMD 14 may deliverstimulation therapy employing housing electrode 36 coupled to theterminal HV-A and at least one electrode such as electrode 32 coupled tothe node HV-B output (at terminals 36 a and 32 a, respectively) of theoutput circuit 56. In alternative embodiments, the IMD 14 may employadditional electrodes such as electrodes 30, 34 coupled to nodes such asS1, S2 (at terminals 30 a and 34 a, respectively) for sensing orstimulation therapy.

The cardioversion-defibrillation stimulation therapy energy andcapacitor charge voltages can be intermediate to those supplied by ICDshaving at least one cardioversion-defibrillation electrode in contactwith the heart and most AEDs having cardioversion-defibrillationelectrodes in contact with the skin. The typical maximum voltagenecessary for ICD 14 using most biphasic waveforms is approximately 750Volts with an associated maximum energy of approximately 40 Joules. Thetypical maximum voltage necessary for AEDs is approximately 2000-5000Volts with an associated maximum energy of approximately 200-360 Joulesdepending upon the waveform used. The SubQ ICD 14 of the presentdisclosure uses maximum voltages in the range of about 700 to about 3150Volts and is associated with energies of about 25 Joules to about 210Joules. The total high voltage capacitance could range from about 50 toabout 300 microfarads.

Such cardioversion-defibrillation stimulation therapies are onlydelivered when a malignant tachyarrhythmia, e.g., ventricularfibrillation is detected through processing of the far field cardiac ECGemploying one of the available detection algorithms known in the ICD 14art.

In FIG. 4, pacer timing/sense amplifier circuit 52 processes the farfield ECG SENSE signal that is developed across a particular ECG sensevector defined by a selected pair of the electrodes 36, 32, andoptionally, electrodes 30, 34 if present as noted above. The selectionof the sensing electrode pair is made through a control circuit 54 in amanner to provide the most reliable sensing of the EGM signal ofinterest, which would be the R wave for patients who are believed to beat risk of ventricular fibrillation leading to sudden death. The farfield ECG signals are passed through the control circuit 54 to the inputof a sense amplifier in the pacer timing/sense amplifier circuit 52.

Control circuit 54 may comprise one or more microprocessors,Application-Specific Integrated Circuits (ASICs), Digital SignalProcessors (DSPs), Field-Programmable Gate Arrays (FPGAs), discreteelectronic components, state machines, sensors, and/or other circuitry.Control circuit 54 may operate under the control of programmedinstructions such as software and/or firmware instructions stored withina storage device (70). The storage device may include volatile,non-volatile, magnetic, optical, and/or electrical media for storingdigital data and programmed instructions, including Random Access Memory(RAM), Read-Only Memory (ROM), Non-Volatile RAM (NVRAM), ElectricallyErasable Programmable ROM (EEPROM), flash memory, removable storagedevices, and the like. These one or more storage devices 70 may storeprograms executed by control circuit 54.

Storage devices 70 may likewise store data, which may include, but isnot limited to, programmed parameters, patient information, data sensedfrom the patient, and status information indicating the status of theICD 14. For instance, the data may include statistical information andother characteristic data about the battery (or individual cell) that isused to predict charge remaining within the power source 46 of ICD 14 aswill be discussed in more detail below. The data may further contain ERIand/or EOL indicators to indicate when replacement operations will beneeded. This information may be provided to a clinician or patient viathe external device 4.

Detection of a malignant tachyarrhythmia is determined via the controlcircuit 54 as a function of one or more sensed signals (e.g., R-wavesignals and/or P-wave signals) that are output from the pacertiming/sense amplifier circuit 52 to the control circuit 54. An exampledetection algorithm is described in U.S. Pat. No. 7,103,404, titled“Detection of Tachyarrhythmia Termination”, issued to Stadler, which isincorporated herein by reference in its entirety. Certain steps in theperformance of the detection algorithm criteria are cooperativelyperformed in a microcomputer 50, including stored detection criteriathat may be programmed into via a telemetry interface (not shown)conventional in the art.

The microcomputer 50 is generally representative of a processor andassociated memory in storage device 70. The memory may reside internallywithin the microcomputer 50, or separately in storage device 53. Thememory, for example, may include computer readable instructions that,when executed by processor, cause the operational circuitry and or anyother component of the medical device to perform various functionsattributed to them. For example, the memory may include any volatile,non-volatile, magnetic, optical, or electrical media, such as a randomaccess memory (RAM), read-only memory (ROM), non-volatile RAM (NVRAM),electrically-erasable programmable ROM (EEPROM), flash memory, or anyother digital media. Such memory will typically be non-transitory. Theprocessor, may include any one or more of a microprocessor, a digitalsignal processor (DSP), a controller, an application specific integratedcircuit (ASIC), a field programmable gate array (FPGA), or equivalentdiscrete or integrated logic circuitry. In one or more exemplaryembodiments, the processor may include multiple components, such as anycombination of one or more microprocessors, one or more controllers, oneor more DSPs, one or more ASICs, or one or more FPGAs, as well as otherdiscrete or integrated logic circuitry. The functions attributed to themicrocomputer 50 may be embodied as software, firmware, hardware, or anycombination thereof.

Data and commands are exchanged between microcomputer 50 and controlcircuit 54, pacer timing/amplifier circuit 52, and output circuit 56 viaa bi-directional data/control bus 61. The pacer timing/amplifier circuit52 and the control circuit 54 are clocked at a slow clock rate. Themicrocomputer 50 is normally asleep, but is awakened and operated by afast clock by interrupts developed by sensed cardiac events or onreceipt of a downlink telemetry programming instruction or upon deliveryof cardiac pacing pulses to perform any necessary mathematicalcalculations, to perform tachycardia and fibrillation detectionprocedures, and to update the time intervals monitored and controlled bythe timers in pace/sense circuitry 52.

The detection algorithms are highly sensitive and specific for thepresence or absence of life threatening ventricular arrhythmias, e.g.,ventricular tachycardia (V-TACH) and ventricular fibrillation (V-FIB).As discussed above, the detection algorithms contemplated in accordancewith this disclosure may utilize sensed cardiac signals to detect thearrhythmias. In addition, detection algorithms for atrial fibrillationmay also be included.

Although the ICD 14 of the present disclosure may rarely be used for anactual sudden death event, the simplicity of design and implementationallows it to be employed in large populations of patients at modest riskwith modest cost by medical personnel other than electrophysiologists.Consequently, the ICD 14 of the present disclosure includes theautomatic detection and therapy of the most malignant rhythm disorders.

When a malignant tachycardia is detected, high voltage capacitors (notshown) within the output circuit are charged to a pre-programmed voltagelevel by a charging circuit 58. It is generally considered inefficientto maintain a constant charge at all times on the high voltagecapacitors. Instead, charging is initiated when control circuit 54issues a high voltage charge command delivered to charging circuit 58and charging is controlled by means of bi-directional signal line(s)from the HV output circuit 56. Without intending to be limiting, thehigh voltage output capacitors may comprise film, aluminum electrolyticor wet tantalum construction. Some examples of the high voltage outputcapacitors are described in commonly assigned U.S. Pat. No. 8,086,312,titled “Capacitors for Medical Devices”, issued to Nielsen, which isincorporated herein by reference in its entirety.

The high voltage output capacitors may be charged to very high voltages,e.g., 700-3150V, to be discharged through the body and heart between theselected electrode pairs among first, second, and, optionally, thirdand/or fourth subcutaneous cardioversion-defibrillation electrodes 36,32, 30, 32. The details of an exemplary charging circuit 58 and outputcircuit 56 will be discussed below. The high voltage capacitors arecharged by charging circuit 58 and a high frequency, high-voltagetransformer. The state of capacitor charge is monitored by circuitrywithin the output circuit 56 that provides a feedback signal indicativeof the voltage to the control circuit 54. Control circuit 54 terminatesthe high voltage charge command when the received signal matches theprogrammed capacitor output voltage, i.e., thecardioversion-defibrillation peak shock voltage.

Control circuit 54 then develops a control signal that is applied to theoutput circuit 56 for triggering the delivery of cardioverting ordefibrillating shocks. In this way, control circuitry 54 serves tocontrol operation of the high voltage output stage 56, which delivershigh energy cardioversion-defibrillation stimulation therapies between aselected pair or pairs of the first, second, and, optionally, the thirdand/or fourth cardioversion-defibrillation electrodes 36, 32, coupled tothe HV-A, HV-B and optionally to other electrodes such as electrodes 34,30 coupled to the S1, S2 terminals as shown in FIG. 4.

Thus, ICD 14 monitors the patient's cardiac status and initiates thedelivery of a cardioversion-defibrillation stimulation therapy through aselected pair or pairs of the first, second, third and/or fourthelectrodes 36, 32, 34, and 30 in response to detection of atachyarrhythmia requiring cardioversion-defibrillation.

Typically, the charging cycle of the capacitors has a short duration,e.g., it can take anywhere from two seconds to about thirty seconds, andoccurs very infrequently. The ICD 14 can be programmed to attempt todeliver cardioversion shocks to the heart in the manners described abovein timed synchrony with a detected R-wave or can be programmed orfabricated to deliver defibrillation shocks to the heart in the mannersdescribed above without attempting to synchronize the delivery to adetected R-wave. Episode data related to the detection of thetachyarrhythmia and delivery of the cardioversion-defibrillationstimulation therapy can be stored in RAM for uplink telemetrytransmission to an external programmer as is well known in the art tofacilitate in diagnosis of the patient's cardiac state.

Housing 14 may include a telemetry circuit (not shown in FIG. 4), sothat it is capable of being programmed by means of external device 4(FIG. 1) via a 2-way telemetry link. Uplink telemetry allows devicestatus and diagnostic/event data to be sent to external programmer forreview by the patient's physician. Downlink telemetry allows theexternal programmer via physician control to allow the programming ofdevice function and the optimization of the detection and therapy for aspecific patient. Programmers and telemetry systems suitable for use inthe practice of the present disclosure have been well known for manyyears. Known programmers typically communicate with an implanted devicevia a bi-directional telemetry link such as Bluetooth®, radio-frequency,near field, or low frequency telemetry link, so that the programmer cantransmit control commands and operational parameter values to bereceived by the implanted device, and so that the implanted device cancommunicate diagnostic and operational data to the programmer.

Those skilled in the art will appreciate that the various components ofthe low power circuit 60 i.e., pacer/sense circuit 52, control circuit54, and microcomputer 50 are illustrated as separate components for easeof discussion. In alternative implementations, the functions attributedto these components 50, 52 and 54 may suitably be performed by a solecomponent.

As mentioned above, the control circuit 54 and output circuit 56performs several functions. One of those is to monitor the state ofcapacitor charge of the high voltage output capacitors. Another functionis to allow the controlled transfer of energy from the high voltageoutput capacitors to the patient.

FIG. 5 illustrates an exemplary schematic showing a portion of theoperational circuitry 48 of FIG. 4, in accordance with an embodiment ofthe disclosure, in greater detail. The output circuit 56 allows thecontrolled transfer of energy from the energy storage capacitors to thepatient 12.

The output circuit 56 includes four legs 80, 82, 84, and 86 that areinterconnected. The interconnection of the four legs with legs 80 and 82being configured in a parallel orientation alongside legs 84 and 86 anda bridge being provided to intersect each of the pair of parallelconnected legs. As is shown in FIG. 5, the interconnected legs arearrayed to define a configuration includes a high side and a low sidethat may resemble a “H”. In other words, the four interconnected legsare arrayed having legs 80 and 84 defining the high side and legs 82 and86 defining the low side.

The intersecting bridge includes HV-A and HV-B terminals that couple theoutput circuit 56 to the cardioversion electrodes 36 and 32. Aspreviously described, patient 12 is connectable (e.g., usingleads/electrodes 36, 32 and any other suitable connections) betweenterminal HV-A located between the switch 80 and switch 82 and terminalHV-B located between switch 84 and switch 86.

Legs 80 and 84 are coupled to a positive terminal of the energy storagecapacitors. An optional discharge switch 88, such as an insulated gatebipolar transistor (IGBT), may be used in the coupling from the legs 80and 84 to the positive terminal of the energy storage capacitors.Discharge switch 88 may be controlled by control circuit 54 (FIG. 4)that is included within the low power circuit 60 to close and remain inthe conducting state during discharge of the capacitors. Leg 82 and 86are coupled to a negative terminal of the energy storage capacitors. Theselection of one or more of the switches 80, 82, 84, 86 under control ofcontrol circuit 54 may be used to provide one or more functions. Forexample, selection of certain switches in one or more configurations maybe used to provide one or more types of stimulation pulses, or may beused to provide active or passive recharge, etc.

For example, in accordance with an embodiment, the ICD 14 provides abiphasic defibrillation pulse to the patient in the following manner.With reference to FIG. 5, once the energy storage capacitors are chargedto a selected energy level, the switches 80, 86, and 88 are closed so asto provide a path from the capacitors to electrode 36, 32 for theapplication of a first phase of a defibrillation pulse to the patient12. The stored energy travels from the positive terminal of thecapacitors, through switch 88 through switch 80, across the patient 12,back through switch 86 to the negative terminal of the capacitors. Thefirst phase of the biphasic pulse therefore applies a positive pulsefrom the electrode 36 to the electrode 32.

After the end of the first phase of the biphasic defibrillation pulse,the switches 88, 84 and 82 are switched on to start the second phase ofthe biphasic pulse. Switches 84 and 82 provide a path to apply anegative defibrillation pulse to the patient 12. With reference to FIG.5, the energy travels from the positive terminal of the capacitors,through switch 88 to switch 84, across the electrodes 32, 36 coupled tothe patient 12, and out through switch 82 to the negative terminal ofthe capacitors. The polarity of the second phase of the defibrillationpulse is therefore opposite in polarity to the first phase of the pulse.

FIG. 6 is a schematic illustrating a portion of the operational circuit48 of IMD 14. As previously mentioned, the operational circuit 48includes at least one power source 46. The power source 46 may comprisea battery having at least two cells 102 a, 102 b (collectively “102”).In exemplary embodiments, the power source 46 may be programmable orstatic and may be a switched or linear regulated source, etc. In anembodiment, the cells 102 supply power to the operational circuit 48 aswell as the charge for stimulation therapy energy. The cells 102 may beformed from materials such as LiCFx, LiMnO2, LiI2, LiSVO or LiMnO2,among others, as is known in the art.

Cell 102 a is coupled to a transformer 64 a and cell 102 b is coupled toa transformer 64 b. The transformers 64 a, 64 b (collectively 64) areincluded within the output circuit 56 (shown in dashed lines in FIG. 6).Transformer 64 a includes a first primary winding 106 a and transformer64 b includes a second primary winding 106 b. In the embodiment, thecell 102 a is coupled to transformer 64 a through the first primarywinding 106 a and the cell 102 b is coupled to the transformer 64 athrough the second primary winding 106 b.

Transformer 64 a includes a core 114 a and a plurality of secondarywindings 116 a-c. The primary winding 106 a is wound around the core 114a and the secondary windings 116 a-c are also wound around the core 114a. The plurality of secondary windings 116 a-c are interlaced along alength of each of the windings 116 a-c. That is, each one of theplurality of secondary windings 116 a-c is interlaced with the otherfive of the plurality of secondary windings 116 a-c.

Transformer 64 b includes a core 114 b and a plurality of secondarywindings 116 d-f. The primary winding 106 b is wound around the core 114b and the secondary windings 116 d-f are also wound around the core 114b. The plurality of secondary windings 116 d-f are interlaced along alength of each of the windings 116 d-f. That is, each one of theplurality of secondary windings 116 d-f is interlaced with the otherfive of the plurality of secondary windings 116 d-f.

A first switching element 108 a is coupled between the first primarywinding 106 a of the transformer 64 a and the cell 102 a. A secondswitching element 108 b is coupled between the second primary winding106 b of the transformer 64 b and the cell 102 a. Each of the switches108 a, 108 b is coupled to a charge monitoring circuit, such as controlcircuit 54 (FIG. 4), which issues control signals (CS1, CS2) toselectively actuate each of the switches 108 a, 108 b. The controlsignals CS1 and CS2 may be the same or different and may be configuredto selectively actuate the switches 108 a, 108 b separately,simultaneously, or in any other desired manner. For example, the controlsignal may be issued as a function of the residual energy of each of thecells 102. In one example, it may be desired that the voltage differencebetween the first and second cells 102 should be within a predeterminedcriteria, such as within a selected range, or no greater than apredetermined voltage value. As such, the total energy in each of thefirst and second cells 102 may be measured and evaluated to determinewhether the predetermined criterion is met. If the criterion is mete.g., difference is within the predetermined range of voltage values,the control signals may be issued to simultaneously couple both cells102 so as to draw energy from both cells 102 simultaneously. However, ifthe criterion is not met, e.g., difference exceeds the predeterminedvoltage value, the control signals may be timed to couple the cells 102at different times so as to draw energy from the cell 102 having thehigher magnitude of energy first until the criterion for the voltagedifference is met.

The cells 102 may be formed such that each cell includes a cathode(positive) terminal and an anode (negative) terminal. As is illustratedin the depicted embodiment, the cathode terminals of cells 102 a, 102 bare coupled to the primary winding 106 a and the primary winding 106 b,respectively, and the anode terminals are both connected to a commonnode, such as the circuit ground node. The switches 108 a, 108 b arealso coupled to the common node. As such, a first circuit path isdefined between the first cell 102 a and first primary winding 106 a anda second circuit path is defined between the second cell 102 b and theprimary winding 106 b. In an alternative embodiment, the anode terminalsof the cells 102 a, 102 b are coupled to the primary winding 106 a andthe primary winding 106 b, respectively, and each of the cathodeterminals is connected to a common node, such as the circuit groundnode.

In one embodiment, the switches 108 are simultaneously actuated to aconducting state to enable current to flow from both cells 102 to thetransformer 64. The actuation of the first switch 108 a into a closedposition triggers charge transfer from the first cell 102 a to the firstprimary winding 106 a and actuation of the second switch 108 b into aclosed position triggers charge transfer from the second cell 102 to thesecond primary winding 106 b. In other words, the closing of switch 108a creates a current path for flow of current from the first cell 102 ato the transformer 106 a while the closing of switch 108 b creates acurrent path for flow of current from the second cell 102 b to thetransformer 106 b.

Each of the secondary windings 116 a-f is coupled to a capacitor forstorage of the charge generated by the transformer 64. Specifically,secondary winding 116 a is coupled to capacitor 122 a, secondary winding116 b is coupled to capacitor 122 b, secondary winding 116 c is coupledto capacitor 122 c, secondary winding 116 d is coupled to capacitor 122d, secondary winding 116 e is coupled to capacitor 122 e, and secondarywinding 116 f is coupled to capacitor 122 f.

The first set of capacitors 122 a-c are coupled in series and the secondset of capacitors 122 d-f are coupled in series. Both sets of capacitors122 a-c and 122 d-f are further coupled together in series.

A diode may optionally be coupled between each of the secondary windingsand the respectively coupled capacitor to bias the flow of current fromthe transformer to each of the capacitors. Specifically, a diode 120 ais coupled between secondary winding 116 a and capacitor 122 a, a diode120 b is coupled between secondary winding 116 b and capacitor 122 b, adiode 120 c is coupled between secondary winding 116 c and capacitor 122c, a diode 120 d is coupled between secondary winding 116 d andcapacitor 122 d, a diode 120 e is coupled between secondary winding 116e and capacitor 122 e, and a diode 120 f is coupled between secondarywinding 116 f and capacitor 122 f. In the embodiment of FIG. 6, thediodes are illustrated for completeness but it should be understood thatin alternate embodiments, the diodes 120 a-f need not be included.

As shown in FIG. 6, the cells 102 are arranged in a parallelconfiguration and each cell 102 a, 102 b simultaneously delivers chargeto its respective primary winding 106 a, 106 b.

Although not shown in the figure, an isolation circuit may be providedto enable coupling of the cells 102 to other circuitry of IMD 14. Suchan isolation circuit enables the cells 102 to deliver current duringhigh power current operations while allowing both cells to contribute tothe current supply to other circuitry such as 50, 52, and 54 ofoperational circuit 48 (shown in FIG. 4) during low power currentoperations. The high power current operations include the delivery ofenergy to the transformer 64 to, for example, provide defibrillationtherapy or other electrical stimulation therapy. The low power currentoperations include supply of power to analog and digital portions of thelow power circuitry 60, for example. For simplicity of description, theinterconnections between the cells 102 and all the components of theoperational circuit 48 is not shown. In the event of a failure of one ofthe cells 102, the isolation circuit 110 isolates the failed cell fromthe other cell.

As previously discussed, the low power circuitry 60 may also includecharge monitoring circuitry (not shown) that is coupled to the outputcircuit 56 to monitor the voltage stored in the capacitors 122. Thevoltage stored in the capacitors 122 corresponds to the voltage that isto be delivered in the form of an electrical stimulation therapy pulseto patient 12. As is known in the art, this voltage may be in the rangeof 200 V to 1800 V.

To facilitate the measurements, the magnitude of the voltage to bemeasured may be reduced to enable the voltage to be measured bycomponents in the charge monitoring circuit that may not be rated tosuch voltages. In one embodiment, a first resistor voltage divider 124 ais utilized for measurement of the cumulative voltage stored by thecapacitors 122 a-c and a second resistor voltage divider 124 b isutilized for measurement of the cumulative voltage stored by thecapacitors 122 d-f. Each resistive voltage divider 124 a, 124 b(collectively “resistive voltage dividers 124”) is coupled in parallelwith the capacitors 122 and an output of the resistive voltage dividers124 is coupled to the charge monitoring circuit. The resistive voltagedividers 124 are configured with each of the dividers 124 having a firstresistor and a second resistor, the values of the first and secondresistors being selected to enable the output voltage generated by theresistive voltage divider 124 to be a fraction of the cumulative voltagestored by the capacitors 122. The resistive voltage dividers 124 andcapacitors 122 are coupled to a common node, in this case the groundnode. The resistor voltage dividers 124 provide a divided voltage value,the divided voltage value being a value that has a magnitude that is afraction of the measured voltage that is stored in the capacitors 122.In other words, the divided voltage value has a magnitude that is lessthan the magnitude of the cumulative voltage that is stored in thecapacitors 122.

The voltage measured by the resistive voltage divider 124 b is measuredwith respect to ground while the voltage measured by the resistivevoltage divider 124 a is also measured with respect to ground. As such,the cumulative voltage of capacitors 122 a-c is obtained by subtractingthe cumulative voltage of capacitors 122 d-f from value measured byresistive voltage divider 124 a.

The two voltage values obtained for the cumulative voltage stored by thecapacitors 122 a-c and the cumulative voltage stored by the capacitors122 d-f are used to enable control of the charging by the transformers64 a, 64 b. The charge monitoring circuit issues the control signal CS1to controls an on-time for delivery of charge from the first cell 102 abased on the value of the output voltage from the resistive voltagedivider 124 a. A second control signal CS2 is issued by the chargemonitoring circuit to controls an on-time for delivery of charge fromthe second cell 102 b based on the value of the output voltage from theresistive voltage divider 124 b. In particular, the control signals CS1,CS2 control the duration of actuation of each of the switches 108 a, 108b.

Providing software, firmware and hardware to accomplish the presentinvention, given the disclosure herein, is within the abilities of oneof skill in the art. For the sake of brevity, conventional techniquesrelated to ventricular/atrial pressure sensing, IMD signal processing,telemetry, and other functional aspects of the systems (and theindividual operating components of the systems) may not be described indetail herein. The connecting lines shown in the various figurescontained herein are intended to represent example functionalrelationships and/or physical couplings between the various elements. Itshould be noted that many alternative or additional functionalrelationships or physical connections may be present in an embodiment ofthe subject matter.

The description refers to elements or nodes or features being“connected” or “coupled” together. As used herein, unless expresslystated otherwise, “connected” means that one element/node/feature isdirectly joined to (or directly communicates with) anotherelement/node/feature, and not necessarily mechanically. Likewise, unlessexpressly stated otherwise, “coupled” means that oneelement/node/feature is directly or indirectly joined to (or directly orindirectly communicates with) another element/node/feature, and notnecessarily mechanically. Thus, although the schematics shown in thefigures depict exemplary arrangements of elements, additionalintervening elements, devices, features, or components may be present inan embodiment of the depicted subject matter.

While the disclosure is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the disclosure to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the disclosure as defined by the appended claims.

What is claimed is:
 1. An implantable medical device, comprising: afirst transformer comprising: a first magnetic core; a first primarywinding disposed around the core; and a first plurality of secondarywindings disposed around the first core and magnetically coupled to thefirst primary winding; a second transformer comprising: a secondmagnetic core; a second primary winding disposed around the second core;and a second plurality of secondary windings disposed around the secondcore and magnetically coupled to the second primary winding; a powersource comprising at least a first cell and a second cell, the firstcell being coupled to the first primary winding and the second cellbeing coupled to the second primary winding; a plurality of capacitorsincluding: a first set of capacitors coupled in a series configuration,wherein each capacitor of the first set of capacitors is coupled acrossonly one secondary winding of the first plurality of secondary windings;and a second set of capacitors coupled in a series configuration,wherein each capacitor of the second set of capacitors is coupled acrossonly one secondary winding of the second plurality of secondarywindings, wherein the first set of capacitors and the second set ofcapacitors are always connected together in a series configuration; anda low power circuit coupled to the power source for controlling thedelivery of charge from the power source to the plurality of capacitorsthrough the first and second transformers.
 2. The implantable medicaldevice of claim 1, wherein the low power circuit includes: a firstresistive voltage divider coupled across the first set of capacitors, asecond resistive voltage divider coupled across the second set ofcapacitors, a charge monitoring circuit coupled to each of the first andsecond resistive voltage dividers, a first switching element coupledalong a first current path of the first cell and the first transformer,and a second switching element coupled along a second current path ofthe second cell and the second transformer.
 3. The implantable medicaldevice of claim 2, wherein the first resistive voltage divider isconfigured to measure a first cumulative voltage across the first set ofcapacitors and to generate a first divided voltage that is a fraction ofthe first cumulative voltage and the second resistive voltage divider isconfigured to measure a second cumulative voltage across the second setof capacitors and to generate a second divided voltage that is afraction of the second cumulative voltage, and wherein the monitoringcircuit generates a first control signal to control an on-time fordelivery of charge from the first cell responsive to the first dividedvoltage and a second control signal to control an on-time for deliveryof charge from the second cell responsive to the second divided voltage.4. The implantable medical device of claim 2, wherein the power source,the first transformer, the second transformer, and the plurality ofcapacitors are coupled to a common node.
 5. The implantable medicaldevice of claim 4, wherein the charge monitoring circuit monitors afirst cumulative voltage across the first set of capacitorsindependently from the monitoring of a second cumulative voltage acrossthe second set of capacitors.
 6. The implantable medical device of claim5, wherein the charge monitoring circuit issues a first control signalto the first switching element and issues a second control signal to thesecond switching element based on the monitored first and secondcumulative voltages, wherein the second control signal is different fromthe first control signal.
 7. The implantable medical device of claim 4,wherein the charge monitoring circuit monitors a first cumulativevoltage across the second set of capacitors measured with respect to thecommon node and monitors a second cumulative voltage across the firstset of capacitors measured with respect to the first cumulative voltageand issues a first control signal for controlling delivery of chargefrom the first cell and a second control signal for controlling thedelivery of charge from the second cell.
 8. The implantable medicaldevice of claim 1, wherein the power source, the first transformer, thesecond transformer, and the plurality of capacitors are coupled to acommon node.
 9. The implantable medical device of claim 8, wherein acharge monitoring circuit monitors a first cumulative voltage across thefirst set of capacitors independently from monitoring of a secondcumulative voltage across the second set of capacitors.
 10. Theimplantable medical device of claim 9, wherein the charge monitoringcircuit issues a first control signal to control charging of the firsttransformer and issues a second control signal to control charging ofthe second transformer based on the monitored first and secondcumulative voltages, wherein the second control signal is different fromthe first control signal.
 11. The implantable medical device of claim 8,wherein a charge monitoring circuit monitors a first cumulative voltageacross the second set of capacitors measured with respect to the commonnode and monitors a second cumulative voltage across the first set ofcapacitors measured with respect to the first cumulative voltage andissues a first control signal for controlling delivery of charge fromthe first cell and a second control signal for controlling the deliveryof charge from the second cell.
 12. The implantable medical device ofclaim 1, further comprising a first switching element configured toselectively couple the first cell to the first primary winding and asecond switching element configured to selectively couple the secondcell to the second primary set.
 13. The implantable medical device ofclaim 12, wherein responsive to a monitoring of a first cumulativevoltage across the first set of capacitors and a second cumulativevoltage across the second set of capacitors, a charge monitoring circuitissues a first control signal and a second control signal, and whereinthe first switching element selectively couples the first cell to thefirst primary winding set for delivery of charge responsive to the firstcontrol signal, and the second switching element selectively couples thesecond cell to the second primary winding set for delivery of chargeresponsive to the second control signal.
 14. The implantable medicaldevice of claim 1, further comprising a diode coupled to each of thewindings of the first and second plurality of secondary winding and eachcapacitor coupled thereto for biasing the flow of current from thetransformer to each of the plurality of capacitors.
 15. The implantablemedical device of claim 1, wherein the first cell comprises a firstcathode and the second cell comprises a second cathode, and wherein thefirst primary winding is directly coupled to the first cathode and thesecond primary winding is directly coupled to the second cathode. 16.The implantable medical device of claim 1, further comprising amonitoring circuit for monitoring a state of charge of each of the firstand second cells.
 17. The implantable medical device of claim 16,wherein the monitoring circuit issues a control signal for controlling arate of cell voltage delivery from each of the first and second cellsand to maintain a voltage difference between the first and second cellswithin a predetermined voltage value.
 18. The implantable medical deviceof claim 16, wherein the monitoring circuit monitors the state of chargethrough coulomb counting.
 19. The implantable medical device of claim16, wherein the monitoring circuit monitors the state of charge throughvoltage measurement.
 20. The implantable medical device of claim 1,wherein the first plurality of secondary windings are interlaced along alength thereof with each other and the second plurality of secondarywindings are interlaced along a length thereof with each other.
 21. Theimplantable medical device of claim 1, wherein the first plurality ofsecondary windings comprises three wires and the second plurality ofsecondary windings comprises three wires.